Friction stir welding (FSW) is an emerging method of fusing a variety of materials and provides numerous advantages over conventional welding methods. FSW is conceptually simple and typically does not require use of any filler metal or flux. As a result, a full strength bond can be achieved with optimal mechanical characteristics. Moreover, as friction stir welding is a solid state fusion process that occurs below the melting point of the material, problems associated with the heat affected zone, unwanted grain growth, shrinkage, and/or distortion can be reduced, if not even entirely avoided. Still further, FSW does in most cases not require post weld treatment. Even more advantageously, FSW provides an efficient and reliable way of joining selected dissimilar metals, and metals and thermoplastic polymers. Exemplary systems and methods of FSW are described in U.S. Pat. Nos. 5,460,317 and 5,794,835. These and all other extrinsic materials discussed herein are incorporated by reference in their entirety.
In practice, a rotating tool (most commonly a non consumable tool (NCT)) rotates at a constant angular velocity and is pressed into the anticipated weld joint line that is formed by the first and second base materials. In most cases, the tool transverses along the joint line at constant velocity, and the frictional heat between the NCT and the first and second materials plasticizes the materials that are then forced around the tool. The transverse movement of the tool allows the mixed material (stir zone) to form a solid state joint (weld). During operation, the first and second base materials that are to be joined are generally held in a fixed position relative to each other to allow proper fusion.
While FSW provides numerous advantages over many conventional welding methods, the stability and integrity of the weld formed by friction stir welding is a function of several process parameters, which may significantly alter the mechanical properties of the solid state fusion. As relatively little is known about the specific contribution of one or more parameters in forming a stable weld between two materials, reliable and reproducible weld formation is often a matter of trial and error and the multitude of possible determinants will often preclude a rationale process design. Consequently, numerous diverse approaches have been undertaken to improve weld integrity. For example, systems and methods for control over the downforce of the tool into the material have been described as one important parameter as reported in U.S. Pat. No. 6,050,475. Similarly, as disclosed in U.S. Pat. No. 7,216,793, a load cell is operationally coupled to the tool such that the travel load can be monitored and maintained constant to so help improve the weld quality. In yet other known methods, specific tool geometry is employed to improve friction and movement of plasticized material as shown in U.S. Pat. No. 7,275,675. Alternatively, improved process and weld control was reported in U.S. Pat. No. 5,829,664 and WO 99/39861 where at least one of the work pieces were preheated. Unfortunately, while such known devices and methods tend to improve certain aspects of FSW, other difficulties often to arise. For example, force controlled systems and methods frequently suffer from relatively slow transverse welding speed, and specific tool design typically limits the transverse force that can be applied, which results in slow welding speeds. On the other hand, where the materials are preheated or heated with an external heater, difficulties may be encountered due to expansion, inadvertent overheating, etc.
Moreover, friction stir welding has generally only found acceptance and practical use with relatively soft or non-ferrous materials (e.g., aluminum, magnesium, copper, zinc, and lead alloys) as these materials become plastic at relatively low temperatures. However, the use of friction stir welding of carbon steels or stainless steel has been limited by the lack of suitable tool materials that can withstand the high temperatures and pressures required to weld such harder materials. Only more recently, tools comprising polycrystalline cubic boron nitride (PCBN) and/or polycrystalline diamond (PCD) have allowed use of FSW with harder materials.
For example, certain carbon steel materials (here: S70C carbon steel) have been welded using FSW by decreasing the peak temperature and by decreasing the cooling rate to less than the lower critical cooling rate as described in Scripta Materialia Volume 56, Issue 7, April 2007, Pages 637-640. Unfortunately, the transverse speed was undesirably low (at 1-4 in/min). Similarly, successful FSW of certain stainless steel materials using a tungsten tool was reported (Materialwissenschaft and Werkstofftechnik, Volume 38 Issue 10, Pages 829-835 (2007)), but again, transverse speed was limited to 1½ to 4 in/min at 1000 rpm. Still
Therefore, even though FSW holds significant promise in joining various materials, numerous questions and concerns still remain. Consequently, there is a need to identify critical FSW process parameters to obtain and optimize welds with predictable and desirable integrity and stability, especially where steel materials are to be welded.